Why are Blackbirds' feet deformed?

Museum curator says cause not known; count shows population drop

Philip Unitt (cq) looks over a bird from the museum's collection of Brewer's blackbirds all showing foot deformities from a mystery ailment. The collection was collected from southern California.
— Nelvin C. Cepeda

Philip Unitt (cq) looks over a bird from the museum's collection of Brewer's blackbirds all showing foot deformities from a mystery ailment. The collection was collected from southern California.
— Nelvin C. Cepeda

Phil Unitt instantly recognized the mangled feet and missing toes as he recently examined a photo of a Mission Valley blackbird.

Unitt, curator of birds and mammals for the San Diego Natural History Museum, has gotten glimpses of the ailment for decades, spotting blackbirds hobbling on deformed feet from San Diego State University to San Francisco. He has even seen museum specimens with the condition, dating to the early 1960s.

And yet Unitt has no idea what causes the disease.

“It is by no means restricted to Mission Valley but is very widespread, in rural areas as well as urban,” Unitt wrote in an e-mail to colleagues about the deformity.

The condition does not debilitate the birds to the point where the public is picking them up and taking them to be rehabilitated. But it may have contributed to the substantial decline of the Brewer’s blackbird population in San Diego County, if not over a wider area, during the past 40 years.

A Mission Valley blackbird with deformed feet caused by a condition that experts know little about. John Tighe

A Mission Valley blackbird with deformed feet caused by a condition that experts know little about. John Tighe

Unitt, author of “San Diego Bird Atlas,” searched unsuccessfully for scientific literature on the ailment while writing that tome. Researchers at the San Diego Zoo’s Institute for Conservation Research also knew about the condition — and were equally baffled by its cause.

Scientists said the illness afflicts the common Brewer’s blackbird, as well as some red-winged and tricolored blackbirds.

It shows up as foot lesions, malformations and eventual loss of toes, and has cropped up throughout San Diego County, the Channel Islands, parts of Riverside County and the Bay Area.

The birds soldier on with missing toes, but it’s unclear whether the condition affects their ultimate survival. Infected birds apparently can still feed and fly, but could be vulnerable to predators if they’re unable to perch.

Unitt and a colleague discussed the disease recently, after uncovering museum specimens of blackbirds with swollen and scaly feet. Then a photo of an afflicted blackbird submitted by a U-T San Diego reader piqued their interest in the mysterious ailment.

In hopes of gaining insight, as well as new samples, Unitt forwarded the photo to Bruce Rideout, director of Wildlife Disease Laboratories at the zoo’s conservation institute, and to Project Wildlife, which rehabilitates sick or injured animals.

Rideout, whose job puts him at the forefront of wildlife pathology, was also perplexed.

Any number of contagions or contaminants could cause the lesions, he wrote in an e-mail to colleagues: avian pox or viral infection with secondary bacterial invasion, scaly leg mites, chronic irritation complicated by bacterial or fungal infections, exposure to caustic chemicals and blood parasites that constrict small blood vessels.

However, he cautioned: “None of these scenarios are a perfect fit with your descriptions and the high (affliction rate) you are seeing.”

To solve the mystery, he said, he’d need to see live birds with sick feet. Molecular analysis of their tissue samples could help pinpoint a cause, he said.

Unidentified wildlife diseases aren’t unheard of, Rideout said. With limited resources, researchers target the most egregious plagues — those that cause mass die-offs or wreak havoc on humans and domestic animals in addition to wildlife. Think West Nile virus and bird flu, which are both potentially lethal to birds and people, or Bluetongue disease, which kills deer and livestock.

“When you look at the landscape of things going on in wildlife, you’re going to focus on things that cause mortality rather than nonfatal foot problems,” Rideout said.

A chronic condition can become catastrophic, however, if other factors — such as changes in habitat, predation or other pathogens — set the stage for decline.

Brewer’s blackbird is a common species that visits suburban lawns, parking lots and shopping malls, Unitt said.

But the bird’s everyday visibility may belie its health. The San Diego Audubon Christmas Bird Count, a popular annual bird survey that Unitt helps coordinate, shows a steady decline. Counts in the 1980s consistently logged more than 1,000 Brewer’s blackbirds, and in some years recorded several thousand of the gregarious, flocking birds.

Starting in the 1990s, the numbers dropped to about 500. They declined to just 231 birds in 2012.

Although it’s unclear whether blackbirds are dwindling across Southern California in general or whether the foot disease plays a role, Unitt said it’s a piece of the puzzle.

In the world of birding, where enthusiasts pursue new and exotic species, the quiet decline of a backyard bird may not raise red flags, he said.

“Because it’s still a common bird, having some diseased individuals is unremarkable,” he said. “No one has been thinking Brewer’s blackbirds could go extinct.”

“The only reason I had looked was in conversation I had heard that there’s this strange deformity, and it was very obvious in these animals,” Tremor said. “They were limping old birds.”

Around the same time, Linda King, a bird keeper for the San Diego Zoo and volunteer with Project Wildlife, delivered a new blackbird carcass to Unitt.

Stretching out its iridescent black wings, Unitt examined the specimen, noting the scaly lesions and swollen joints on its feet, one of which was also tangled with purple thread.

“It’s remarkable that this condition has been going on for 50 years in Southern California, but it has never been studied,” he said.

Philip Unitt (cq) looks over a samples from the museum's collection of Brewer's blackbirds all showing foot deformities from a mystery ailment. The collection was collected from southern California.
— Nelvin C. Cepeda

Philip Unitt (cq) looks over a samples from the museum's collection of Brewer's blackbirds all showing foot deformities from a mystery ailment. The collection was collected from southern California.
— Nelvin C. Cepeda